The easy answer is how many new customers or additional sales will it take to cover the cost of the new website? You may also consider how many existing customer or sales will you not lose if you develop or redesign your site.
For example, if your average profit on the sale of your products or servies is $200 and the cost of your new website is $1400, you will need just 7 new sales transactions or keep from losing 7 sales transactions to cover the cost of your investment.